It could be any of a number of problems. A heater that's working properly should blow air that's about 75-100 degrees F. hotter than the outside air. If it doesn't, any of the following might be at fault:

* A low coolant level, often due to a leak or weak radiator cap, but may also be the result of not getting the cooling system completely filled.

* An open thermostat that's too cold for the application. Most vehicles today require a 190-195 degree F. thermostat.

* A defective heater control valve.

* A plugged heater core.

* An inoperative airflow control or inlet door in the heater ducting or plenum.

* A defective blower motor.

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